Optical Absorption and Emission in Rare Earth Heavy Metal Fluoride Glasses
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OPTICAL ABSORPTION AND EMISSION IN RARE EARTH HEAVY METAL FLUORIDE GLASSES R. PAFCHEK, J. ANIANO, E. SNITZER AND G.H. SIGELJr. Rutgers University, Fiber Optics Materials Research Program, Box 909, Piscataway, N.J. 08854 ABSTRACT Trivalent thulium is a 4-level laser system which operates in the 1.471tm spectral region. A tunable amplifier at this wavelength is of great interest since it falls in one of the telecommunication windows. For lasing action to occur the relatively long lived 3F 4 lower laser level must be quenched to eliminate the self-terminating behavior of thulium. Rosenblatt et al. co-doped Tm and Th into a ZBLAN host to quench the 3 F4 lower laser level.[1] Co-doping with Ho offers the possibility of more efficient laser operation, but it must be established whether Ho can effectively quench the lower level of Tm. This preliminary study indicated appreciable energy transfer occurred from the 3 F4 level of Tm to the 517 level of Ho which decreased the lower laser level lifetime by as much as two orders of magnitude. The subsequent decay of the Ho ions is an issue requiring further investigation. INTRODUCTION Erbium is capable of lasing in the telecommunications window at 1.5ý1m, but operates as a 3-level system. Four-level systems have inherent advantages over 3-level systems in that the former systems are length independent and less sensitive to the aperture intensity distribution. In addition, 4-level systems allow the possibility of utilizing a double clad fiber configuration. This allows easier coupling with the pump and the option of pumping with multiple stripe laser diodes. Recently 4-level pulsed laser action at 1.47p.m has been reported by Rosenblatt et al. who utilized Tm as the active ion.[2] Terbium was co-doped into the ZBLAN host glass to quench the 3F 4 lower laser level. However, substantial energy transfer also occurred from the 3H4 upper laser level, which decreased the efficiency. Holmium, on the other hand, may provide an alternate means of depleting the 3F 4 lower level of Tm while minimizing the decrease in the lifetime of the upper 3H4 level. This reduction in upper level quenching is due to the absence of a overlap between the Tm fluorescence and Ho absorption near 1.47p1m. Figures 1 and 2 demonstrate the various energy transitions for both the Tm-Tb and Tm-Ho systems.
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I
S3HI e----... .
I
I
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I
AT.3*
T-TI.
N*•
TnO.
Fig I Energy level transitions for Tm - Tb
Fig. 2 Energy level transitions for Tm
Mat. Res. Soc. Symp. Proc. Vol. 172. q1990 Materials Research Society
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Ho
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One possible problem with this scheme is a build up of energy in the 5I7 Ho level. The addition of a small amount of a third ion or possibly simultaneous lasing involving the 517 - 518 transition in Ho may deplete this level. Lasing of this transition in a fluoride glass host has been demonstrated by Brierley et al. at 2.081im.[3] EXPERIMENTAL PROCEDURE Twenty-five gram batches were prepared from certified optical grade Air Product raw materials. The base glass composition in mole perc
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